1. Technical Field of the Invention
This invention is related to magnetic flux leakage testing for flaws in well tubing.
2. State of the Prior Art
Oil and gas wells, water wells, injection wells, and other kinds of wells drilled into the ground are often lined with pipe casing that is cemented into the well hole to protect the well hole from pressures and debris and to isolate various geological strata and fluid zones from other over-laying and under-laying strata and fluid zones. Many of such wells are completed for production with tubing strings, e.g., lengths of smaller diameter pipes (tubing) fastened together and lowered into the well through the casing. Various items of completion equipment can be included in the tubing strings, such as packers, bridge plugs, valves, pumping apparatus, and other devices, depending the particular characteristics and functions of the well. Other well completion steps, such as perforating the casing into reservoir production zones, hydraulic fracturing, work-over cementing and re-perforating into new reservoir zones, and the like can be performed through the tubing. In oil and gas wells, the oil and gas produced from various production zones in the well may be flowed through the tubing to the surface of the well. In injection wells, water or other fluids may be injected into subsurface reservoirs through the tubing. Over time, flaws can develop in such tubing due to rust or other corrosion, fatigue, mechanical impacts, scratches, gouges, wear, and other instrumentalities that can be manifested, for example, in pits, burrs, holes, cracks, reduced wall thicknesses, and other forms of metal loss or abnormalities on inside surfaces of the tubing, outside surfaces, or both. Such flaws, if severe enough, can degrade the integrity of the tubing and potentially result in failure, for example, leaks, bursting, collapse, or breakage, which can adversely affect the operation of the well or cause expensive shutdowns for repairs.
Magnetic flux leakage testing is a common technique used for non-invasive inspection of steel and other kinds of ferromagnetic metal pipes to find flaws or defects before they become severe enough to cause failure. The terms “flaw” and “defect” are synonymous for this description and may be used interchangeably According to the technique, a powerful magnet is used to magnetize portions of the ferromagnetic pipe or tubing, preferably, but not necessarily, to magnetic saturation. Damaged or flawed areas of the tubing in which there has been metal loss, such as the flaws discussed above, cannot support as much magnetic flux as undamaged areas. Therefore, at locations in the ferromagnetic tubing wall where there are flaws, the magnetic field “leaks” from the ferromagnetic material and can be detected and measured. Hall effect transducers, coils, or other sensors positioned adjacent the magnetically saturated tubing can be used to produce a voltage that is indicative of the strength of the magnetic field, thus can detect and measure perturbations in the magnetic field adjacent the tubing caused by such magnetic flux leakage.
Shallow reading sensors (sometimes called wall sensors) used in conjunction with the magnetic flux leakage detectors can be used to determine whether the detected flaw is on the tubing surface that is adjacent to the magnetic flux detectors or on the opposite surface. Shallow reading sensors detect flaws in the surface adjacent to the magnetic flux leakage detectors. Therefore, if a flaw is sensed by both the magnetic flux leakage detector and the shallow reading detector, the flaw is deduced to be located in the surface of the ferromagnetic tubing adjacent to the magnetic flux leakage detectors. However, if a flaw is sensed by the magnetic flux leakage detector but not by the shallow reading detector, then the flaw can be deduced to be located in the opposite surface of the ferromagnetic tubing. Typical shallow reading sensors include reluctance sensors and eddy current sensors. An example reluctance detector may comprise a coil and a small magnet combination that responds to changes in the magnetic field resulting from a perturbing defect or flaw on the tubing surface adjacent the magnetic flux leakage detectors. Hall effect sensors can also be used to as reluctance sensors. An example eddy current sensor for sensing eddy currents caused by flaws on the surface of the ferromagnetic material that is adjacent to the magnetic flux detectors may include a coil that detects magnetic field effects induced by the eddy currents resulting from the flaws on the surface of the ferromagnetic tubing.
Configurations of magnetic flux leakage detector apparatus are available commercially for use inside pipes and tubings, for example, the U.S. Pat. No. 6,924,640, which are sometimes called internal magnetic flux leakage detectors. Other configurations of magnetic flux leakage detector apparatus are available for use on the outsides of pipes and other ferromagnetic objects, sometimes called external magnetic flux leakage detectors. Internal magnetic flux leakage detector apparatus generally comprise a magnetic field generator, magnetic leakage detectors, shallow reading detectors, and data collection circuits packaged together in a module that is sized and shaped for traveling through the interior of pipe to be inspected, such as pipelines, well casings, well tubing strings, and the like. The magnetic field generator creates a strong magnetic field to magnetize the pipe from the inside, and the sensors detect the perturbations in the magnetic field as the apparatus moves through the pipe to detect flaws in the pipe as explained above. External magnetic flux leakage detector apparatus generally comprise similar magnetic field generator, sensor, and data collection components, but are shaped for travel along outer surfaces of pipes, so the magnetization of the pipe and detector sensing is provided adjacent to the outside surfaces of the pipe.
Well production tubing must be pulled out of wells occasionally (sometimes called “tripping pipe” or “tripping tubing”) for various maintenance, repairs, re-completions to different reservoir zones, and other purposes. Therefore, external magnetic flux leakage detection equipment has been developed and used for performing magnetic flux leakage inspection for flaws on such well tubing as the well tubing is being pulled out of the well, so that individual pipe or tubing sections that have flaws of a sufficient severity to be problematic can be identified and marked or set aside. Some of such magnetic flux leakage inspection equipment is designed and configured for setting on a workover or pulling rig floor above the well head in a position for the rig to pull the pipe or tubing sections through the magnetic flux leakage detector apparatus as the pipe or tubing is pulled out of the well.
The foregoing examples of related art and limitations related therewith are intended to be illustrative, but not exclusive or exhaustive, of the subject matter. Other aspects and limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.